1. Field of the Invention
This invention relates in general to the field of marine oil and gas well drilling and in particular to a valve placed in a deep water riser string to prevent collapse of the riser tube by the pressure of the sea water when the internal pressure of the riser drops, for example due to expansion of rising formation gas or loss of circulation and subsequent reduction height of the mud column. Another purpose of this invention is to facilitate opening, by remote control, of the bore at the lower end of the riser so that drill cuttings can be discharged.
2. Description of the Prior Art
One type of prior art marine anti-collapse valve includes a plurality of ports in a riser pup joint which are normally closed by an internal sleeve. The sleeve moves upwardly to uncover the ports when a pressure sensor is activated because the pressure inside the riser falls below a predetermined level below the ambient ocean pressure. Such a valve, may be self-contained and independent of any control lines, or it can be hydraulically operated through control lines to the surface.
A marine riser anti-collapse valve described in the Composite Catalog published by World Oil (1978-79) at page 6857 by Vetco Offshore, Inc., includes a plurality of ports in a riser pup joint normally covered by an external sleeve. A differential pressure sensor is mounted on the riser joint below the sleeve. The pressure sensor, a balanced spool type hydraulic valve, senses a difference in hydrostatic head of a preset value between the inside and the outside of the riser. If the internal pressure drops below the external hydrostatic pressure, the pressure sensor shifts position causing a hydraulic control system to apply hydraulic supply to open four hydraulic cylinders linked to the external sleeve. As a result, the sleeve slides up permitting sea water to enter the riser via the ports. Springs maintain the cylinder in the closed position until their force is overcome by the hydraulic supply applied to the cylinders.
Another external sliding sleeve anti-collapse valve or automatic riser fill valve includes two hydraulic overrides which permit the valve to be opened or closed as desired.
A problem common to the prior art sliding sleeve type anti-collapse valves is the difficulty of providing reliable seals between the sliding sleeves and the ports in the riser pup joint wall. Such seals, must be of a complex geometry by virtue of their position on the cylindrical wall of the riser pup joint.
Another disadvantage with prior art riser anti-collapse valves described above concerns the complexity of hydraulic control systems necessary for their operation, particularly with designs which include a diaphragm sensor valve.
Still another disadvantage of the use of diaphragm sensor valves in prior art anti-collapse valves has been that they occasionally have been overly sensitive to a small region of pressure drop within the bore of the riser in proximity to the diaphragm sensor. Anti-collapse valves with such a design may be opened unintentionally by a swabbing action within the riser pup joint where for example, the drilling string is being raised from the borehole.
Another disadvantage of the prior art anti-collapse riser valves described above is that they are inherently slow to open or close by virtue of the massive sleeves used to uncover or cover ports in the body wall of the pup joint.
Another disadvantage of the prior art anti-collapse riser valves described above concerns difficulty of their assembly and repair. Seals and cylinders must fit over riser couplings or the riser pup joint must have a mechanism for it to be disconnected along its body wall. But such a disconnection mechanism inherently presents problems of assembly and preloading necessary for adequate strength and fatigue resistance in order for the riser pup joint to serve as a marine riser tensile member.
Another disadvantage of prior art anti-collapse valves have been fouling or plugging of the sensing means, such as a diaphragm, by solidified drilling mud as solid fragments.
Still another disadvantage of such prior art designs concerns the high expense of a large cylindrical sliding sleeve and a mechanism for disassembling the valves for seal replacements.